Later this week, NASA plans to launch its fifth Mars rover, Perseverance, on a six-month journey to the Red Planet. Perseverance will boot up a mission to collect samples of Martian dirt that might have traces of ancient life, so that they can be returned to Earth by another mission later this decade. It will also carry a payload unlike anything that’s ever been boosted into space: a small autonomous helicopter called Ingenuity. Sometime next spring, probably in April, Ingenuity will spin up its rotor blades and become the first spacecraft to go airborne on Mars.
“I see it as kind of a Wright brothers moment on another planet,” says Bob Balaram, the chief engineer for the Mars helicopter project at NASA’s Jet Propulsion Laboratory. “It’s a high-risk, high-reward mission that could enable us to go to lots of places we haven’t been able to go before.”
Satellites are good at getting a global understanding of a planet, and the rovers are great at exploring a relatively small amount of terrain in minute detail. For everything in between, it helps to have an airborne system. A rover can only cover a few dozen kilometers over the course of several years, but future extraterrestrial drones could easily cover that in a day. They could take aerial snapshots to help a rover plot the best path or collect samples and return them to a stationary lander for analysis. Ingenuity won’t be able to do any actual science, but it’s the first step toward an extraterrestrial aircraft that can.
Ingenuity’s hardware—cameras, communications equipment, avionics—is stuffed in a small cube that will be suspended in the air by four spindly legs that make it look a bit like a robotic insect. Up top, there are two pairs of rotor blades, each four feet in diameter, sandwiched between Ingenuity’s body and a rectangular solar panel. The whole apparatus weighs less than a full two-liter soda bottle, but it's hardy enough to withstand the extreme environments it will face during launch, landing, and its day-to-day existence on the Martian surface.
Once Perseverance arrives on Mars, it will spend a few weeks checking out its systems. If everything looks good, its first order of business will be to find a clearing in the rock-strewn Jezero crater to drop off its passenger. (And it will literally be dropped—the helicopter is attached to the belly of the rover.) Once the rover and the helicopter part ways, the chopper’s days are numbered. Balaram and his team will only have a month to conduct up to five test flights. “The whole intent of this campaign is to get engineering data so we can say this worked the way we thought and there were no surprises on Mars,” says Balaram. “Beyond 30 days, we’d just be a distraction.”
Like the Wright brothers’ famous flight test at Kitty Hawk, on its first flight Ingenuity will only be in the air for a few seconds. This hop will be a nearly exact replica of flight tests Balaram and his crew did back on Earth so they can make an apples-to-apples comparison of the helicopter’s performance against expectations. If everything goes well, Ingenuity will attempt increasingly challenging flight profiles. The helicopter is designed to fly up to 15 feet in the air and can travel up to three football fields from its takeoff point. Its batteries limit it to just 90 seconds of flight time, but this will be more than sufficient for the types of flight demos it will do on Mars.
For Balaram, the first Martian flight has been a long time coming. He cooked up a plan for an extraterrestrial chopper in the late 1990s—although the idea wasn’t exactly new—after seeing a conference presentation by Ilan Kroo, an aerospace engineer at Stanford University who had spent the past few years working on a coin-sized atmospheric research drone called the mesicopter. As Kroo and his team knew all too well from their research, aerodynamics becomes soupy at small scales, which makes controlling flight difficult. "We soon realized that flying mesi-scale devices on earth was very similar, at least aerodynamically, to flying larger vehicles on Mars," says Kroo. "We started working with Bob Balaram and the Jet Propulsion Lab to take our tiny rotor designs and scale them up to fly on Mars."
Balaram and Kroo submitted a proposal for a Mars helicopter to NASA in the early 2000s, but the proposal was never funded despite positive feedback from reviewers. (Balaram blames budget cuts at the agency.) The idea languished on the shelf for another 15 years until Charles Elachi, the director of NASA’s Jet Propulsion Laboratory, asked Balaram to rework the proposal and submit it as a possible ridealong experiment for the agency’s newest rover. In 2018, NASA officials announced that the helicopter would be the scientific sideshow on the Mars 2020 mission. By that point, R&D on the chopper was well underway.
NASA tapped AeroVironment, a drone manufacturer in California, to build the hardware for the mission. The company has a lot of experience operating autonomous aircraft in extreme environments—and a bit of history with NASA. In 2001, the company contracted with the agency to build a solar-powered drone that managed to fly at 96,000 feet; 20 years later, the record still stands. That altitude on Earth is comparable to flying near the surface on Mars because of the planet’s tenuous atmosphere. But flying a small helicopter on Mars makes piloting a giant solar powered wing on Earth look easy.
“We had to keep everything super lightweight to make the whole program work,” says Ben Pipenberg, an aeromechanical engineer at AeroVironment. “We really tried to pull every milligram out of every single component, because that’s really what it takes to get the weight low enough to fly on Mars”
The Ingenuity team had to balance the stringent weight requirements with competing demands on durability and performance. Even though the chopper’s weight was capped at four pounds, it had to be strong enough to withstand the intense forces it would encounter during launch and landing. Its hardware also had to meet the demands of the mission, like having a motor that can spin the rotor blades five times faster than a typical helicopter so it can generate lift. Oh, and it will need a computer powerful enough to run the machine vision algorithms the helicopter will use to autonomously navigate the Martian landscape. It’s a lot to ask of a machine that weighs less than a laptop.
“This pushed every single technical discipline,” says MiMi Aung, the project manager for Ingenuity at NASA’s Jet Propulsion Laboratory. “There were a lot of unnerving moments.”
To trim weight, engineers at AeroVironment made the blades out of foam and wrapped them in carbon fiber, and used more exotic materials, like beryllium metal matrix composites, for other body components. For avionics and power supply, the team turned to commercial off-the-shelf parts. Ingenuity stores its power with a common lithium ion battery and its computer is a Qualcomm Snapdragon processor, which is found in a variety of smartphones. They might not be quite as immune to failure as the hardware on the Perseverance rover, but they’re cheaper than using space-grade hardware while also meeting the helicopter’s performance requirements. Since Ingenuity isn’t critical to the rover’s main mission, the JPL team could afford to take a chance on some smartphone components.
There was also the challenge of simply figuring out how to test the thing. “Nobody has done this before, so the team had to invent a way to incrementally test the vehicle while another team is inventing the helicopter in parallel,” says Aung. “We were really paranoid, and we had to be, because we were under a lot of time pressure to progress fast enough to catch the rover launch. So we really had to think ahead.”
The team built two prototypes of Ingenuity: one for environmental testing and the other for flight tests. Environmental testing is the art of making life hell for a spacecraft. A prototype of Ingenuity was exposed to extremely cold temperatures to mimic conditions on Mars, it was placed near small detonations to make sure it could withstand the explosive shocks from the charges on the rover used to deploy the landing parachute, and it was blasted with the biggest and baddest stereo system around to see if all its nuts and bolts will hold tight when exposed to the extreme vibrations of a rocket launch.
The flight tests took place in a giant 25-foot diameter vacuum chamber that was pumped full of carbon dioxide to replicate the composition and thinness of the Martian atmosphere. The gravity on Mars is only about one-third as strong as on Earth, and since NASA hasn’t yet figured out how to manipulate gravity itself, the agency’s engineers have to compensate in other ways to create a realistic Martian scenario. For Ingenuity, this meant attaching a gravity offload tether to the vehicle. The tether looks a bit like fishing line and can be dynamically adjusted to pull up on the helicopter just enough to simulate the effects of reduced gravity while it’s flying.
As far as physics is concerned, flying a helicopter on Mars is fundamentally the same as flying a helicopter on Earth: The blades spin and pull air downward fast enough to generate lift. But the devil is in the details, and hands-on flight experiments helped the Ingenuity team discover some quirks about flying a chopper on another planet. During one early test, an AeroVironment engineer found that he was able to flawlessly pilot an Ingenuity prototype in an open vacuum chamber. But once the chamber was sealed and the air pumped out to replicate Martian conditions, the helicopter started behaving erratically and became difficult to fly. “That’s when we realized maybe the control isn’t as straightforward as we think it is,” says Balaram.
On Earth, helicopter blades have a natural tendency to flap as they rotate due to the length of the blades and the turbulent aerodynamic environment around the rotor. The feedback from this flapping would make a helicopter nearly impossible to control if it weren’t for the fact that the Earth’s thick atmosphere damps the vibrations to a manageable level. But as the AeroVironment engineer discovered, Mars’s atmosphere is too thin to have this flap damping effect, and as this ripples through the machine it wreaks havoc on its controls. “This had all our NASA helicopter experts tremendously excited, because to them it was like seeing everything with fresh new eyes,” says Balaram. To compensate for this effect, the Ingenuity team rebuilt the blades to make them stiffer.
There’s a lot riding on the accuracy of the flight test results. Unlike the Ingenuity prototypes, which have logged hours of flight time, the helicopter headed to Mars has only spent a few minutes in the air on Earth. “We didn’t want to wear out the system in the process of testing it,” says Balaram. By the time NASA abandons Ingenuity on the Martian surface, the intrepid little helicopter will have flown for fewer than 30 minutes.
Balaram says that NASA is already working on the next generation of extraterrestrial choppers, and the engineering data collected by Ingenuity during its flight tests will directly affect their development. These future helicopters may look a lot different than Ingenuity—one design NASA is studying has six rotors, for instance—and they’ll certainly be larger.
But the first scientific flight on another planet may not happen on Mars. In 2025, NASA plans to send a small nuclear-powered quadcopter called Dragonfly on a mission to hunt for life around Titan, Saturn’s largest moon. Dragonfly will be much longer-lived than Ingenuity—it’s expected to spend two years hopping around on the moon’s surface—and it will have a 2 mile altitude range. Titan is generally considered to be the easiest place to fly in the solar system because of its extremely dense atmosphere and low gravity. “You could strap on wings and fly there yourself, if you didn’t mind the cold,” Balaram says. For now, though, we’ll have to make do with a helicopter.
Updated 7-28-2020, 10:15 am ET: Perseverance is NASA's fifth Mars rover.